This invention relates generally to boiling water reactors and more particularly, to a forged upper shroud section for a shroud of such a reactor.
A reactor pressure vessel (RPV) of a boiling water reactor (BWR) typically has a generally cylindrical shape and is closed at both ends, e.g., by a bottom head and a removable top head. A top guide, sometimes referred to as a grid, typically is spaced above a core plate within the RPV. A core shroud, or shroud, typically surrounds the core plate and is supported by a shroud support structure. Particularly, the shroud has a generally cylindrical shape and surrounds both the core plate and the top guide. The top guide includes several openings, and fuel bundles are inserted through the openings and are supported by the core plate.
The shroud, due to its large size, is formed by welding a plurality of stainless steel cylindrical sections together. Specifically, respective ends of adjacent shroud sections are joined with a circumferential weld. A typical arrangement for the upper shroud section is a welded assembly composed of a top guide grid, a shroud flange and a cylinder section. The grid is a mesh of plates located at a lower end of the upper shroud section and typically is fabricated by welding plates to a cylindrical rim. The grid provides horizontal support for the fuel assemblies.
The cylinder of the upper shroud section typically is fabricated from formed and welded plates. The cylinder provides a barrier to separate the upward flow through the separator from the downward flow in the annulus between the shroud and the reactor pressure vessel, provides structural support between the grid and shroud flange, and creates a space/volume between the top of the fuel and the bottom of the shroud head.
The shroud flange is a circular flange located on the top of the upper shroud section. The flange typically is fabricated from several plate sections which are joined by welding. The flange provides a surface/structure to which the shroud head may be bolted, provides structural support of the shroud head, and provides a sealing surface between the shroud and shroud head to limit steam from exiting the shroud.
In more recent known configurations, grids are fabricated from single piece or two piece forgings. The shroud flange and cylinder, however, continue to be fabricated using welded plates or forgings. The use of forgings has decreased the number of welds, but several welds remain in the upper shroud section.
The shroud welds, however, increase the susceptibility of the shroud material to a detrimental effect known as inter-granular stress corrosion cracking (IGSCC). Typically, cracking may occur in the heat affected zone of the shroud welds. Eliminating welds in the upper shroud section would eliminate the potential of cracks initiating in welds.
These and other objects may be attained by a forged upper shroud section which may be machined from a single piece rectangular cross-section ring forging and includes a circular flange and a cylindrical shell. Openings and slots are machined into the flange to align and support the shroud head. A groove is machined along an inside surface of the cylinder section, and the groove may be used to support top guide grid (not shown). An end of the cylinder section is machined with a weld prep for attachment to the core section of the shroud.
The above described upper shroud section is fabricated from a single piece forging and therefore, fewer welds are required with such upper shroud section as compared to known upper shroud section. In addition, the present upper shroud section provides the same flow barrier as the welded upper sections, provides a flange to which the shroud head may be bolted and supported, provides a groove to which the top guide grid may be attached without the need for a ledge or flange, and the number to total shroud welds is reduced because of the single piece design. Reducing the number of welds minimizes cracking which can occur in shroud welds and also reduces the number of welds which must be inspected during the construction and life of the shroud.